1. Field of the Invention
This invention relates to integrated circuit (IC) fabrication, and more particularly relates to a method of patterning a target layer on a substrate, which can make a reduced device pitch.
2. Description of the Related Art
As the requirement on the integration degree of IC devices becomes higher, the device pitch has to be reduced. The device pitch can be reduced by enhancing the lithographic resolution, or in an easier way by forming spacers at two sides of each photoresist pattern and removing the photoresist patterns so that the remaining spacers have a reduced pitch and serve as an etching mask for the target layer to be patterned.
FIGS. 1A-1F illustrate a method of patterning a target layer in a reduced pitch in the prior art.
Referring to FIG. 1A, a target layer 104 to be patterned and a photoacid chemical amplification photoresist layer 106 are formed on a substrate 102. An exposure step is done to form unexposed parts 106a and exposed parts 106b in the photoresist layer 106, wherein each exposed part 106b has a low proton (photoacid) concentration and each unexposed part 106a has a very low proton concentration in a top portion and sidewalls thereof. Post-exposure baking (PEB) is performed causing chemical amplification of the photoacid, so that the exposed parts 106b have a high proton concentration and each unexposed part 106a has a low proton concentration in its top portion and sidewalls.
Referring to FIG. 1B, a development step is performed to remove the exposed parts 106b, such that only the unexposed parts 106a remain on the target layer 104.
Referring to FIG. 1C, an acid-crosslinked polymer layer 112 is formed on the top portion and the sidewalls of each unexposed part 106a. The acid-crosslinked polymer layer 112 is usually formed by coating a layer of acid-crosslinkable polymeric material, baking the same to cause crosslinkage around each unexposed part 106a by the protons at its top portion and sidewalls and removing the non-crosslinked polymeric material. This step called a RELACS process, as described in U.S. Pat. No. 6,383,952.
Referring to FIG. 1D, the top portion of the acid-crosslinked polymer layer 112 over each unexposed part 106a is removed by etching-back or chemical mechanical polishing (CMP) to form polymer spacers 112a, wherein the surface layer 106c of each unexposed part 106a is chemically altered due to the etching-back or CMP effect.
Referring to FIG. 1E, a flood exposure step 114 and another PEB step 116 are performed in turn so that the previously unexposed parts 106a all have a high proton concentration and can be dissolved in a development liquid.
Referring to FIG. 1F, another development step is done to remove the previously unexposed parts 106a and form patterns (112a) with a reduced pitch. However, for the surface layers 106c of the unexposed parts 106a have been chemically altered, they are not all removed completely in the development, as shown in FIG. 1F. Hence, the target layer 104 is patterned with the polymer spacers 112a and the residual unexposed parts 106a and 106c as a mask to have different pitches, as indicated by the dash lines.